There are many applications giving rise to the need to control the voltage to an electrical load powered by an alternating current voltage source. One such application is phase-controlling dimmer switches which are used in conjunction with incandescent lighting to vary the intensity of light provided. Known systems typically utilize electronic switches such as silicon-controlled rectifiers (SCR's) or TRIAC's to limit the phase angle conduction of AC voltage to a controlled AC load. It is this process that is known as phase control.
It is well known to implement phase control to a load from an AC circuit in this manner by using electronic switches that turn "on" to a conductive condition at a pre-selected time delay after the zero crossing of the AC periodic wave form and then commute "off" to a blocking condition at the next zero crossing of the waveform when the current through the devices decays to zero. This method of conventional or "forward" phase control is required due to TRIAC's and SCR's ability to be turned on by applying a gate pulse, whereas they can only be turned "off" when the current they are conducting is removed. A disadvantage to forward phase control however, is that the load current undergoes a dramatic rise over a relatively small time interval (di/dt) when the switching device is turned "on" to conduction. This is due to the almost instantaneous application of up to 172 volts (peak voltage in a 120 volt RMS system) across the load.
In the case of an incandescent lamp, this surge in current through the lamp filament will create an intense magnetic field which will cause the filament and its supports to change their length. This change is known as magnetostriction and can shake the entire lamp. It is this phenomenon which is known as incandescent lamp "hum" and when either the incandescent lamp is of a high enough wattage or is placed in an environment with very low ambient sound can produce an audible sound that is unacceptable. In addition this large di/dt can cause Radio Frequency Interference which interferes with audio equipment and AM radio reception. Clearly, this too, is unacceptable.
To minimize the effects of this dramatic increase in current and voltage over such a short period of time, circuits have been developed which use current chokes to limit the current rise to acceptable levels. One such current choke used by manufacturers is to place a large inductor in series with the load. The inductor can then limit the rate of change of current passing through the load and reduce the electromagnetic interference. In the case of an incandescent lamp, an inductor can decrease lamp "hum".
A problem with this practice, however, is that since the inductor is placed in series with the load, it carries the entire load current. As a result, the inductor serves to detract power from the load. Typically, the voltage drop across the inductor can be as much as several volts. Other problems are that the inductors themselves can produce audible noise from their cores, as well as heat, they are physically large, and they are relatively expensive.
More recent systems developed to afford reverse phase control, use pairs of voltage-controlled devices such as metal-oxide-semiconductor field-effect-transistor switches (MOSFET's) and insulated-gate bipolar transistors (IGBT's). These devices have the ability to be either turned "on" or "off" through control of their gate voltage. These devices, therefore, are typically configured using reverse phase control, whereby these devices do not delay turn on into the AC half-cycle but enable conduction immediately following zero-crossing of the AC line during the leading edge of the AC cycle and are turned "off" after some predetermined time, but before the next zero-crossing In this manner, these devices significantly reduce di/dt to an acceptable level at turn-on. Turn "off" di/dt, however, must still be controlled. Toward this end various methods and circuits have been disclosed in U.S. Pat. Nos. 4,540,893, 4,547,828, 4,823,069, and 4,528,494 which serve to reduce di/dt and incandescent lamp hum at turn "off".
All of these known circuits, however, rely on essentially the same method for enabling the voltage-controlled switches. That is, except for U.S. Pat. No. 4,823,069, the references teach treating the pair of switches as one unit, developing a zero-crossing detection mechanism, enabling the switches ostensibly at zero-crossing of the AC waveform, and terminating conduction after same pre-defined time interval. While U.S. Pat. No. 4,823,069 does not teach treating the switch unit, since redundant circuitry is employed for each switch, each switch is still enabled following some defined interval after zero-crossing.
The most prevalent problem associated with these prior art methods of reverse phase control is that of accurate zero-crossing detection which is crucial to the goal of minimal noise and interference generation. If the voltage controlled switches in a zero-crossing detection circuit are not enabled based on an accurately detected zero-crossing, the switches will serve to generate additional electromagnetic interference and incandescent lamp hum which are precisely the phenomena that the circuit is designed to prohibit.
While some known circuits, such as that disclosed in U.S. Pat. No. 4,823,069 employ controlled rise time circuitry to address this problem, the circuitry adds complexity and, inevitably, expense, to the circuit. Additionally, the additional energy absorbed by the controlled rise time circuitry will be dissipated as heat. These complicated circuits still do not address, however, the problem of inaccurately determined zero-crossing detection leading to an erratic phase angle of conduction of the voltage controlled switches. This inevitably leads to instability in the output voltage of the entire system.
Various phenomena contribute to the difficulty of accurate zero-crossing detection. One such factor is that as the waveform of the line voltage approaches zero, any line noise can disturb accurate detection of a zero voltage condition. This problem is particularly germane in commercial environments. While known mechanisms such as filters and damping networks could be used to minimize ambient noise, such solutions would also serve to delay zero crossing detection and thereby resulting in the problems discussed above. Another problem associated with zero-crossing detection is that due to its analog nature, comparators, diodes, and various other analog components are required. These components will add cost and complexity to a system and often require a pulse shaping device, such as that taught by U.S. Pat. No. 4,528,494 in order to interface with any other digital components of the circuit.
It is therefore an object of the present invention to provide a method and circuit for reverse phase control which eliminates the need for accurate zero crossing detection.
It is another object of the present invention to provide a method and circuit for controlling the conduction period of two voltage controlled switches by operating the switches in an inverse fashion.
It is yet another object of the present invention to provide a novel approach to the use of a reliable, stable triggering mechanism for a time base to measure conduction phase angle.
It is still another object of the present invention to provide a method and circuit for reverse phase angle control which can be easily incorporated into digitally controlled or microcontroller controlled applications.